The present invention relates to the art of combustion control and combustion control systems. The present invention finds particular application in an apparatus and method for controlling the temperature in a batch-loaded furnace heated with oxygen enriched air burners and will be described with particular reference thereto. It is to be appreciated, however, that the invention is applicable to the control of other types of furnaces and finds use for oxygen enriched air combustion systems which operate over a wide range of heat output rates.
Heretofore, combustion rates and furnace temperatures have been controlled by increasing and decreasing both air and fuel flow rates in a fixed, stoichiometric ratio. For example, air was commonly supplied to the combustion nozzle or burner at 9.41 times the supply rate of Toledo natural gas. As more calories of heat were required, the fuel and air supply rates were both increased while maintaining the 9.41:1 ratio. Similarly, when less heat was required, both flow rates were reduced in this fixed proportion.
For some applications, it has been found to be advantageous to supply the burner with oxygen enriched air to substantially improve the furnace efficiency. That is, oxygen gas has been mixed with the air to increase its natural 21% oxygen concentration to a higher level, e.g., 45%. The stoichiometric combustion ratio for oxygen-enriched air with a 45% oxygen concentration and Toledo natural gas is 4.34:1. To increase or decrease the combustion rate, the supply of fuel and the supply of oxygen-enriched air were increased or deceased, respectively, together such that the stoichiometric combustion ratio remained constant over all combustion levels.
One of the problems encountered with the prior art combustion control systems was that at low heat inputs, there were correspondingly low flow rates of air or enriched air and fuel supplied to the burner. These low flow rates resulted in a lazy, low velocity flame which lacked sufficient momentum to distribute the heat uniformly. A wide variation in the combustion rate produced correspondingly wide variations in combustion momentum, hence in temperature distributions. Prior art furnaces which were operated at near 100% combustion capacity to bring the treated objects up to temperature and at a relatively low percent of capacity to maintain the temperature were susceptible to temperature gradients at the lower combustion rates.
Further, the prior art combustion control systems experienced more difficulty controlling combustion rates at low percentages of capacity. At relatively low combustion levels, the air and fuel pressures were relatively low, rendering precise control elusive.
The present invention contemplates a new and improved control system for oxygen-enriched burners which overcomes the above-referenced problems and others.